The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting on a cellular phone, a digital still camera, a portable information terminal, a security camera, an overhead camera, a scanner, a network camera, and the like.
With advancement in ICT (Information and Communication Technology), information and knowledge are increasingly shared. In these years, ICT devices, which are developed based on the ICT, have been brought. Among them, an overhead camera, so-called a document camera, which can enlarge and project objects including from a two-dimensional object such as textbooks and reference materials to three-dimensional objects, have been widely used mainly in educational sites and conference rooms. Since such a document camera can enlarge and display a reference material or an actual object as is, for example, in case of use in a conference room, a speaker can make variations in a presentation flexibly, and thereby listeners can easily understand contents of the presentation and see an actual object as being projected.
Since a document camera is usually placed on a desk, it is preferred to have a small size and also to be able to project or display more information finely, which includes letters and figures. For this reason, in addition to a small size, an imaging lens for mounting on the document camera is required to have a high resolution and a wide imaging angle of view so as to attain a wide imaging range.
However, it is difficult to attain miniaturization while satisfactorily correcting aberrations for such a high resolution and also attain the wide imaging angle of view. For example, if a size of an imaging lens is reduced, refractive power of each lens needs to be strong, so that it is difficult to satisfactorily correct aberrations. Accordingly, upon actual designing of an imaging lens, it is a key to meet those requirements in a balanced manner.
As an imaging lens with a wide imaging angle of view, for example, the one described in Patent Reference has been known. The imaging lens includes a front group having negative refractive power and a rear group having positive refractive power, arranged in the order from an object side. The front group includes a first lens having a biconcave shape and a second lens that has a shape of a meniscus lens directing a concave surface thereof to the object side and is negative. The rear group includes a third lens having a biconvex shape, a fourth lens having a biconcave shape, and a fifth lens having a biconvex shape.
According to the configuration described above, it is possible to restrain an increase in distortion accompanied by widening the imaging angle of view while attaining such widening, by restraining a ratio of a composite focal length of the front group to a focal length of the whole lens system and a ratio of a composite focal length of the rear group to the focal length of the whole lens system respectively within preferred ranges.    Patent Reference Japanese Patent Publication No. 2009-134175
In the imaging lens for mounting on such a document camera, it is strongly required to have an ability of exactly imaging an object to project. According to the imaging lens described in Patent Reference, although it has the wide angle of view in a diagonal direction, which is as wide as about 130°, because of relatively large distortion, it is difficult to exactly image the object.
Such an issue is not specific to the imaging lens for mounting on the document camera, and is common among imaging lenses for mounting on relatively small cameras such as digital still cameras, portable information terminals, security cameras, or network cameras.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that can satisfactorily correct distortion in spite of a wide angle of view thereof.